Systems and methods for implementing magnetoelectric junctions

ABSTRACT

Embodiments of the invention implement DIOMEJ cells. In one embodiment, a DIOMEJ cell includes: an MEJ that includes, a ferromagnetic fixed layer, a ferromagnetic free layer, and a dielectric layer interposed between said fixed and free layers, where the fixed layer is magnetically polarized in a first direction, where the free layer has a first easy axis that is aligned with the first direction, and where the MEJ is configured such that when a potential difference is applied across it, the magnetic anisotropy of the free layer is altered such that the relative strength of the magnetic anisotropy along a second easy axis that is orthogonal to the first easy axis, as compared to the strength of the magnetic anisotropy along the first easy axis, is magnified for the duration of the application of the potential difference; and a diode, where the diode and the MEJ are arranged in series.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application claims priority to U.S. Provisional ApplicationNo. 61/698,635, filed Sep. 8, 2012, the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to the implementation ofmagnetoelectric junctions.

BACKGROUND OF THE INVENTION

Devices that rely on the interplay between electricity and magnetismunderlie much of modern electronics. Relatively recently, researchershave begun to develop and implement such devices that take advantage ofquantum mechanical magnetoresistance effects, such as giantmagnetoresistance (GMR) and tunnel magnetoresistance (TMR). GMR and TMRprinciples regard how the resistance of a thin film structure thatincludes alternating layers of ferromagnetic and non-magnetic layersdepends upon whether the ferromagnetic layers are in a parallel orantiparallel alignment. For example, magnetoresistive random-accessmemory (MRAM) is a technology that is being developed that typicallyutilizes TMR phenomena in providing for alternative random-access memory(RAM) devices. In a typical MRAM bit, data is stored in a magneticpolarization within an arrangement that includes two ferromagneticplates separated by an insulating layer—this arrangement isconventionally referred to as a magnetic tunnel junction (MTJ). One ofthe ferromagnetic plates (the fixed layer) is permanently set to aparticular polarization, while the other ferromagnetic plate (the freelayer) can have its magnetic polarization altered. Generally, the MRAMbit can be written to by manipulating the magnetic polarization of thefree layer such that it is either parallel or antiparallel with thepolarization of the fixed layer; and the bit can be read by measuringits resistance, since the resistance of the bit will depend on whetherthe polarizations are in a parallel or antiparallel alignment.

MRAM technologies initially exhibited a number of deficiencies. Inparticular, the bits tended to be inefficient since they required arelatively large current to manipulate the magnetic polarization of thebit's free layer. Consequently, adjunct technologies were implemented tomitigate these deficiencies. For example, spin-transfer torque MRAM(STT-MRAM) is a variant of the base MRAM technology whereby themagnetizing current constitutes spin-aligned electrons that are used todirectly torque the domains. Additionally, Thermal Assisted SwitchingMRAM (TAS-MRAM) is yet another variant of MRAM technology whereby theMTJs are heated during the write phase; the heating of the MTJs reducesthe current required to polarize the free layer.

Nonetheless, in spite of these advances to MRAM technology and in spiteof the many potential advantages that MRAM technology offers, it has yetto be made to be commercially viable. Accordingly, there exists a needto develop more effective electromagnetic configurations that implementmagnetoresistance principles such that they can be made to be morecommercially viable.

SUMMARY OF THE INVENTION

Systems and methods in accordance with embodiments of the inventionimplement DIOMEJ cells that include a diode arranged in series with amagnetoelectric junction (MEJ). In one embodiment, a DIOMEJ cellincludes: a magnetoelectric junction, that itself includes aferromagnetic fixed layer, a ferromagnetic, magnetically anisotropic,free layer, and a dielectric layer interposed between said ferromagneticfixed layer and ferromagnetic, magnetically anisotropic, free layer,where the ferromagnetic fixed layer is magnetically polarized in a firstdirection, where the ferromagnetic, magnetically anisotropic, free layerhas a first easy axis that is substantially aligned with the firstdirection, such that the ferromagnetic, magnetically anisotropic, freelayer can adopt a magnetic polarity that is either parallel with orantiparallel with the first direction, and where the magnetoelectricjunction is configured such that when a potential difference is appliedacross the magnetoelectric junction, the magnetic anisotropy of theferromagnetic, magnetically anisotropic, free layer is altered such thatthe relative strength of the magnetic anisotropy along a second easyaxis that is orthogonal to the first easy axis, or the easy plane wherethere is no easy axis that is orthogonal to the first easy axis, ascompared to the strength of the magnetic anisotropy along the first easyaxis, is magnified or reduced for the duration of the application of thepotential difference; and a diode, where the diode and themagnetoelectric junction are arranged in series.

In another embodiment, the first direction coincides with an in-planedirection.

In yet another embodiment, the first direction coincides with anout-of-plane direction.

In even another embodiment, the coercivity of the ferromagnetic,magnetically anisotropic, free layer is reduced when a potentialdifference is applied across the magnetoelectric junction.

In still another embodiment, the application of a first thresholdpotential difference across the ferromagnetic fixed layer and theferromagnetic, magnetically anisotropic, free layer reduces thecoercivity of the ferromagnetic, magnetically anisotropic, free layer toan extent where the strength of the magnetic field imposed by theferromagnetic fixed layer is sufficient to magnetize the ferromagnetic,magnetically anisotropic, free layer.

In even yet another embodiment, the application of a second thresholdpotential difference that is greater in magnitude than the firstthreshold potential difference causes a spin-transfer torque current toflow through the magnetoelectric junction, where the spin-transfertorque current magnetizes the ferromagnetic, magnetically anisotropic,free layer in a direction antiparallel with the first direction.

In a further embodiment, the ferromagnetic fixed layer includes one of:iron, nickel, manganese, cobalt, FeCoB, FeGaB, FePd, and FePt.

In a yet further embodiment, the ferromagnetic, magneticallyanisotropic, free layer includes one of: iron, nickel, manganese,cobalt, FeCoB, FeGaB, FePd, and FePt.

In an even further embodiment, the dielectric layer comprises one of:MgO and Al2O3.

In a still further embodiment, a DIOMEJ cell further includes anexternally applied magnetic field that is either parallel with orantiparallel with the magnetic polarization of the ferromagnetic fixedlayer, wherein the externally applied magnetic field has a strengthsufficient to magnetize the ferromagnetic, magnetically anisotropic,free layer when its coercivity is reduced with the application of apotential difference across the ferromagnetic fixed layer and theferromagnetic free layer.

In an even yet further embodiment, a DIOMEJ cell further includes a seedlayer.

In another embodiment, the seed layer comprises Tantalum.

In yet another embodiment, a DIOMEJ cell further includes a seconddielectric layer and a semi-fixed layer, where the second dielectriclayer is interposed between the ferromagnetic, magnetically anisotropic,free layer and the semi-fixed layer; where the semi-fixed layer has adirection of magnetic polarization that is antiparallel with thedirection of magnetic polarization of the ferromagnetic fixed layer; andwhere, when a potential difference is applied across the magnetoelectricjunction, the magnetic anisotropy of the semi-fixed layer is alteredsuch that the relative strength of the magnetic anisotropy along a thirdeasy axis that is orthogonal to the first easy axis, or the easy planewhere there is no easy axis that is orthogonal to the first easy axis,as compared to the strength of the magnetic anisotropy along the firsteasy axis, is magnified or reduced for the duration of the applicationof a potential difference; where the extent of this alteration is lessthan that of the ferromagnetic, magnetically anisotropic, free layer.

In even another embodiment, the application of a potential differencepulse that has a duration that coincides with half of the precessionalperiod of the ferromagnetic, magnetically anisotropic, free layer, or anodd multiple thereof, inverts the direction of magnetic polarization ofthe magnetoelectric junction.

In a further embodiment, a magneto-electric random access memory,includes: an array of DIOMEJ cells; where each DIOMEJ cell includes: amagnetoelectric junction, that itself includes: a ferromagnetic fixedlayer; a ferromagnetic, magnetically anisotropic, free layer; and adielectric layer interposed between said ferromagnetic fixed layer andferromagnetic, magnetically anisotropic, free layer; where theferromagnetic fixed layer is magnetically polarized in a firstdirection; where the ferromagnetic, magnetically anisotropic, free layerhas a first easy axis that is substantially aligned with the firstdirection, such that the ferromagnetic, magnetically anisotropic, freelayer can adopt a magnetic polarity that is either parallel with orantiparallel with the first direction; and where the magnetoelectricjunction is configured such that when a potential difference is appliedacross the magnetoelectric junction, the magnetic anisotropy of theferromagnetic, magnetically anisotropic, free layer is altered such thatthe relative strength of the magnetic anisotropy along a second easyaxis that is orthogonal to the first easy axis, or the easy plane wherethere is no easy axis that is orthogonal to the first easy axis, ascompared to the strength of the magnetic anisotropy along the first easyaxis, is magnified or reduced for the duration of the application of thepotential difference; and a diode; where the diode and themagnetoelectric junction are arranged in series; a plurality of sourcelines; and a plurality of bit lines; where each DIOMEJ cell iselectrically connected to a unique combination of a source line and abit line, such that no other DIOMEJ cell is connected to the same bitline and the same source line; and where a source line and a bit linecan be used to establish a potential difference across a particularDIOMEJ cell.

In a yet further embodiment, for at least one DIOMEJ cell, the firstdirection coincides with an in-plane direction.

In a still yet further embodiment, for at least one DIOMEJ cell, thefirst direction coincides with an out-of-plane direction.

In an even further embodiment, for at least one DIOMEJ cell, theapplication of a first threshold potential difference across theferromagnetic fixed layer and the ferromagnetic, magneticallyanisotropic, free layer reduces the coercivity of the ferromagnetic,magnetically anisotropic, free layer to an extent where the strength ofthe magnetic field imposed by the ferromagnetic fixed layer issufficient to magnetize the ferromagnetic, magnetically anisotropic,free layer.

In an even yet further embodiment, for the at least one DIOMEJ cell, theapplication of a second threshold potential difference that is greaterin magnitude than the first threshold potential difference causes aspin-transfer torque current to flow through the magnetoelectricjunction that magnetizes the ferromagnetic, magnetically anisotropic,free layer in a direction antiparallel with the first direction.

In a still even yet further embodiment, for at least one DIOMEJ cell,the coercivity of the ferromagnetic, magnetically anisotropic, freelayer is reduced when a potential difference is applied across themagnetoelectric junction.

In another embodiment, for at least one DIOMEJ cell, the application ofa potential difference pulse that has a duration that coincides withhalf of the precessional period of the ferromagnetic, magneticallyanisotropic, free layer, or an odd multiple thereof, inverts thedirection of magnetic polarization of the magnetoelectric junction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an MEJ that includes in-plane anisotropies inaccordance with embodiments of the invention.

FIG. 2 illustrates an MEJ that includes out-of-plane anisotropies inaccordance with embodiments of the invention.

FIG. 3 illustrates an MEJ that includes adjunct layers to facilitate itsoperation in accordance with embodiments of the invention.

FIGS. 4A and 4B illustrate MEJs that include a semi-fixed layer inaccordance with embodiments of the invention.

FIGS. 5A and 5B illustrate the operation of an MEJ in accordance withembodiments of the invention.

FIG. 6 illustrates a DIOMEJ cell that incorporates a Schottky diode inaccordance with embodiments of the invention.

FIG. 7 illustrates a DIOMEJ cell that incorporates a diode-type devicein accordance with embodiments of the invention.

FIG. 8 illustrates a DIOMEJ cell where the diode is in electricalcontact with the free layer of an MEJ in accordance with embodiments ofthe invention.

FIG. 9 illustrates a 3D schematic of a DIOMEJ cell in accordance withembodiments of the invention.

FIG. 10 illustrates a DIOMEJ cell that has a circular cross-sectionalshape in accordance with embodiments of the invention.

FIGS. 11A-11C illustrate different shapes that a DIOMEJ cell may adoptin accordance with embodiments of the invention.

FIG. 12 illustrates a MeRAM configuration that employs DIOMEJ cells inaccordance with embodiments of the invention.

FIG. 13 illustrates a configuration that implements stacked arrays ofDIOMEJ cells in accordance with embodiments of the invention.

FIG. 14 illustrates a field programmable gate array that includes DIOMEJcells in accordance with embodiments of the invention.

FIG. 15 illustrates the layered deposition of a DIOMEJ cell inaccordance with embodiments of the invention.

FIG. 16 illustrates implementing a hard mask during the manufacture of aDIOMEJ cell in accordance with embodiments of the invention.

FIG. 17 depicts DIOMEJ cells that have been defined by an etchingprocess during manufacture in accordance with embodiments of theinvention.

FIG. 18 illustrates the deposition of a spacer layer during themanufacture of a DIOMEJ cell in accordance with embodiments of theinvention.

FIG. 19 illustrates the deposition of an oxide or nitride during themanufacture of a DIOMEJ cell in accordance with embodiments of theinvention.

FIG. 20 illustrates the opening of an oxide or nitride during themanufacture of a DIOMEJ cell in accordance with embodiments of theinvention.

FIG. 21 illustrates the deposition of an electrode in accordance withembodiments of the invention.

FIG. 22 illustrates a process for manufacturing a DIOMEJ cell inaccordance with embodiments of the invention.

DETAILED DESCRIPTION

Turning now to the drawings, systems and methods for implementing DIOMEJcells that include a magnetoelectric junction arranged in series with adiode are illustrated. Previous efforts at implementing electromagneticcomponents that utilize magnetoresistance phenomena to achieve twoinformation states (i.e. one bit of information), e.g. MTJs, werelargely directed at using a current to produce a magnetic field tomanipulate the device. However, the currents required were oftenconsiderable, e.g., in cases where MTJs were used in MRAMconfigurations. Indeed, in applications that require low-poweroperation, the requirement of a considerable current made theimplementation of devices that rely on MTJs less commercially viable.Accordingly, voltage-controlled magnetic anisotropy-based MTJs (VMTJs)that generally allow MTJs to utilize an electric field to facilitate theimposition of a magnetic polarization onto the free layer as opposed to(or in some cases, in addition to) using a current to do so weredeveloped and reported. See e.g., International Patent ApplicationNumber PCT/US2012/038693, International Publication Number WO2012/159078 A2, entitled “Voltage-Controlled Magnetic Anisotropy (VCMA)Switch and Magneto-electric Memory (MERAM),” by Khalili Amiri et al.,the disclosure of which is herein incorporated by reference. It has beendemonstrated that such devices result in marked performance improvementsover conventional MTJs. In the instant application, the termThagnetoelectric junction' (MEJ) is used to refer to devices that useVoltage-Controlled Magnetic Anisotropy (VCMA) principles to help themrealize two distinct information states, e.g. voltage-controlledmagnetic anisotropy-based MTJs (VMTJs) as well as the VCMA switchesdisclosed in International Patent Application Number PCT/US2012/038693,cited above. In many instances, an MEJ includes a ferromagnetic fixedlayer, a ferromagnetic, magnetically anisotropic, free layer, and adielectric layer interposed between said ferromagnetic fixed layer andferromagnetic, magnetically anisotropic, free layer. The ferromagneticfixed layer has a fixed magnetic polarization, whereas theferromagnetic, magnetically anisotropic, free layer can be magnetizedsuch that it has a polarization either parallel with or antiparallelwith the ferromagnetic fixed magnet. In many instances, the applicationof a potential difference across the MEJ, allows the free layer to bemagnetized in a desired direction; the free layer can thereby bemagnetized either parallel with or antiparallel with the polarity of thefixed magnet. In accordance with magnetoresistance principles, theresistance of the MEJ will vary depending upon whether the free layeradopts a parallel magnetic polarization or an antiparallel magneticpolarization, and therefore, the MEJ can define two information states(i.e. one bit of information). Thus, MEJs can utilize VCMA principles tohelp them achieve two distinct information states.

Conventional devices that rely on magnetoresistance principles foroperation typically utilize transistors as access devices, e.g. tosupply current. Transistors have been used, in part, because of theirability to supply current of opposing polarities, which has beenrequired to write different bits of information in many conventionaldevices. However, transistors are typically relatively bulky.Accordingly, systems and methods in accordance with embodiments of theinstant invention implement configurations whereby voltages of a singlepolarity can be used to alter the logic state of an MEJ, such that adiode, which can be less bulky than a transistor, can be used as theMEJ's access device; a configuration that includes such an MEJ with adiode acting as an access device is referred to as a DIOMEJ cell. Theutilization of diodes, as opposed to transistors, as an access devicecan confer many advantages. For instance, as can be inferred, DIOMEJcells can be more densely arranged than MEJ-transistor components. Thus,in an ‘MRAM’ type array of MEJ bits, i.e. magnetoelectric RAM (MeRAM),where a crossbar arrangement is implemented, the implementation ofDIOMEJ cells can allow for the 3D stacking of crossbars, therebyincreasing the bit density of the MeRAM configuration. Such aconfiguration is more viable when using DIOMEJ cells as opposed toMEJ-transistor components. Moreover, using diodes as the access device,can eliminate or reduce undesirable sneak currents that are typicallypresent in traditional crossbar arrays.

Accordingly, in many embodiments of the invention, a DIOMEJ cellincludes: a magnetoelectric junction, that itself includes aferromagnetic fixed layer, a ferromagnetic, magnetically anisotropic,free layer, and a dielectric layer interposed between said ferromagneticfixed layer and ferromagnetic, magnetically anisotropic, free layer,where the ferromagnetic fixed layer is magnetically polarized in a firstdirection, where the ferromagnetic, magnetically anisotropic, free layerhas a first easy axis that is substantially aligned with the firstdirection, such that the ferromagnetic, magnetically anisotropic, freelayer can adopt a magnetic polarity that is either parallel with orantiparallel with the first direction, and where the magnetoelectricjunction is configured such that when a potential difference is appliedacross the magnetoelectric junction, the magnetic anisotropy of theferromagnetic, magnetically anisotropic, free layer is altered such thatthe relative strength of the magnetic anisotropy along a second easyaxis that is orthogonal to the first easy axis, or the easy plane wherethere is no easy axis that is orthogonal to the first easy axis, ascompared to the strength of the magnetic anisotropy along the first easyaxis, is magnified or reduced for the duration of the application of thepotential difference; and a diode, where the diode and themagnetoelectric junction are arranged in series. The MEJ structure isnow discussed in greater detail.

Structures for Magnetoelectric Junctions

In many embodiments of the invention, a DIOMEJ cell includes amagnetoelectric junction (MEJ). Any suitable MEJ can be used—forexample, any of the MEJs disclosed in International Patent ApplicationNumber PCT/US2012/038693, cited above, can be implemented. In manyembodiments, the MEJ includes a ferromagnetic (FM) fixed layer, an FM,magnetically anisotropic, free layer (for simplicity, the terms “FM,magnetically anisotropic, free layer” and “FM free layer” will beconsidered equivalent throughout this application, unless otherwisestated), and a dielectric layer separating the FM fixed layer and FMfree layer. Generally, the FM fixed layer has a fixed magneticpolarization, i.e. the direction of magnetic polarization of the FMfixed layer does not change during the operation of the MEJ. Converselythe FM free layer can be magnetized such that it has a polarizationeither parallel with or antiparallel with the FM fixed layer, i.e.during the normal operation of the MEJ, the direction of magnetizationcan be made to change. For example, the FM free layer may have amagnetic anisotropy, whereby it has an easy axis that is substantiallyaligned with the direction of magnetic polarization of the FM fixedlayer. The easy axis refers to the axis, along which, there is atendency for the layer to magnetize. In other words, an easy axis is anenergetically favorable direction (axis) of spontaneous magnetizationthat is determined by the sources of magnetic anisotropy listed below.Relatedly, an easy plane is a plane whereby the direction ofmagnetization is favored to be within the plane, although there is nobias toward a particular axis within the plane. The easy axis and thedirection of magnetic polarization are considered to be ‘substantiallyaligned’ when the polarization of the FM free layer can be made to be atleast partially parallel or antiparallel to the direction of magneticpolarization of the FM fixed layer to the extent that the principles ofmagnetoresistance occur and result in a distinct measurable differencein the resistance of the MEJ as between when the magnetic polarizationsof the FM layers are parallel relative to when they are antiparallel,e.g. such that two distinct information states can be defined.

The principles of voltage-controlled magnetic anisotropy (VCMA) can berelied on in switching the FM free layer's characteristic magneticpolarization, i.e. the application of a potential difference between theFM fixed layer and the FM free layer generally augments the FM freelayer's direction of magnetic anisotropy, and relatedly reduces itscoercivity. Accordingly, with a reduced coercivity, the FM free layercan be subject to magnetization that can make it parallel with orantiparallel with the direction of magnetic polarization for the FMfixed layer. A more involved discussion regarding the general operatingprinciples of an MEJ is presented in the following section.

Notably, the direction of magnetic polarization, and the relatedcharacteristics of magnetic anisotropy, can be established for the FMfixed and FM free layers using any suitable method. For instance, theshapes of the constituent FM fixed layer, FM free layer, and dielectriclayer, can be selected based on desired magnetic polarizationorientations. For example, implementing FM fixed, FM free, anddielectric layers that have an elongated shape, e.g. have an ellipticalcross-section, may tend to induce magnetic anisotropy that is in thedirection of the length of the elongated member—i.e. the FM fixed and FMfree layers will possess a tendency to be magnetized in the directionalong the length of the elongated member. In other words, the directionof the magnetic polarization is ‘in-plane’. Alternatively, where it isdesired that the magnetic anisotropy have a directional component thatis perpendicular to the FM fixed and FM free layers (i.e.,‘out-of-plane’), the shape of the layers can be made to be symmetrical,e.g. circular, and further the FM layers can be made to be thin. In thiscase, while the tendency of the magnetization to remain in-plane maystill exist, it may not have a preferred directionality with in theplane of the layer, and thus the layer may define an easy plane insofaras there is an anisotropic tendency within the plane of the layeralthough there is no preferred axis of magnetization within the plane.Where the FM layers are relatively thinner, the anisotropic effects thatresult from interfaces between the FM layers and any adjacent layers,which tend to be out-of-plane, may tend to dominate the overallanisotropy of the FM layer. Alternatively, a material may be used forthe FM fixed or free layers which has a bulk perpendicular anisotropy,i.e. an anisotropy originating from its bulk (volume) rather than fromits interfaces. The FM free or fixed layers may also consist of a numberof sub-layers, with the interfacial anisotropy between individualsub-layers giving rise to an effective bulk anisotropy to the materialas a whole. Alternatively, FM free or fixed layers may be constructedwhich combine these effects, and for example have both interfacial andbulk contributions to perpendicular anisotropy.

FIG. 1 illustrates an MEJ whereby the FM fixed layer and the FM freelayer are separated by, and directly adjoined to, a dielectric layer. Inparticular, in the illustrated embodiment, the MEJ 100 includes an FMfixed layer 102 that is adjoined to a dielectric layer 106, therebyforming a first interface 108; the MEJ further includes an FM free layer104 that is adjoined to the dielectric layer 106 on an opposing side ofthe first interface 108, and thereby forms a second interface 110. TheMEJ 100 has an FM fixed layer 102 that has a magnetic polarization 112that is in-plane, and depicted in the illustration as being from left toright. Accordingly, the FM free layer is configured such that it canadopt a magnetic polarization 114 that is either parallel with orantiparallel with the magnetic polarization of the FM fixed layer.Additional contacts (capping or seed materials, or multilayers ofmaterials, not shown) may be attached to the FM free layer 104 and theFM fixed layer 102, thereby forming additional interfaces. The contactsboth contribute to the electrical and magnetic characteristics of thedevice by providing additional interfaces, and can also be used to applya potential difference across the device. Additionally, it should ofcourse be understood that MEJs can include metallic contacts that canallow them to interconnect with other electrical components.

Importantly, by appropriately selecting the materials, the MEJ can beconfigured such that the application of a potential difference acrossthe FM fixed layer and the FM free layer can modify the magneticanisotropy of the FM free layer. For example, whereas in FIG. 1, themagnetic anisotropy of the FM free layer is depicted as being in-plane,the application of a voltage may distort the magnetic anisotropy of theFM free layer such that it includes a component that is at leastpartially out of plane. The particular dynamics of the modification ofthe magnetic anisotropy will be discussed below in the section entitled“MEJ Operating Principles.” Suitable materials for the FM layers suchthat this effect can be implemented include iron, nickel, manganese,cobalt, FeCoB, FeGaB, FePd, and FePt; further, any compounds or alloysthat include these materials may also be suitable. Suitable materialsfor the dielectric layer include MgO and Al₂O₃. Of course, it should beunderstood that the material selection is not limited to thoserecited—any suitable FM material can be used for the FM fixed and freelayers, and any suitable material can be used for the dielectric layerin accordance with embodiments of the invention. It should also beunderstood that each of the FM free layer, FM fixed layer, anddielectric layer may consist of a number of sub-layers, which actingtogether provide the functionality of the respective layer.

FIG. 2 illustrates an MEJ whereby the orientation of the magneticpolarizations is perpendicular to the plane of the constituent layers.In particular, the MEJ 200 is similarly configured to that seen in FIG.1, with an FM fixed layer 202 and an FM free layer 204 adjoined to adielectric layer 206. However, unlike the MEJ in FIG. 1, the magneticpolarizations of the FM fixed and FM free layers, 212 and 214respectively, are oriented perpendicularly to the layers of the MEJ.Additional contacts (capping or seed materials, or multilayers ofmaterials, not shown) may be attached to the FM free layer 204 and theFM fixed layer 202, thereby forming additional interfaces. The contactsboth contribute to the electrical and magnetic characteristics of thedevice by providing additional interfaces, and can also be used to applya potential difference across the device. It should also be understoodthat each of the FM free layer, FM fixed layer, and dielectric layer mayconsist of a number of sub-layers, which acting together provide thefunctionality of the respective layer.

Of course, it should be understood that the direction of magneticpolarization for the FM layers can be in any direction in accordancewith embodiments of the invention, as long as the FM free layer canadopt a direction of magnetic polarization that is either parallel withor antiparallel with the direction of magnetic polarization of the FMfixed layer, or contains a polarization component that is eitherparallel or anti-parallel with the direction of magnetic polarization ofthe FM fixed layer. For example, the direction of magnetic polarizationcan include both in-plane and out-of-plane components.

Indeed, it has been observed that where the in-plane and out-of-planeanisotropies are relatively similar, thereby resulting in an overallanisotropy that has anisotropic components in-plane and out-of-plane, anMEJ is most sensitive to VCMA principles and can thereby beadvantageous.

In many embodiments, an MEJ includes, in addition to an FM fixed layer,an FM free layer, and a dielectric layer, additional adjunct layers thatfunction to facilitate the operation of the MEJ. For example, in manyembodiments, the FM free layer includes a capping or seed layer, whichcan help induce greater electron spin perpendicular to the surface ofthe layer and/or can enhance the sensitivity to the application of apotential difference.

FIG. 3 illustrates an MEJ 300 that includes multiple layers that work inaggregate to facilitate the functionality of the MEJ 300. A pillarsection 302 extends from a planar section 304. A voltage is shown beingapplied 306 between the top and bottom of the pillar. By way of example,an Si/SiO2 substrate 308 is seen over which is a bottom electrode 310.The pillar 302 comprises the following layers in order: Ta 312 (e.g., 5nm in thickness); a free layer 314 preferably comprising an Fe-richCoFeB material (e.g. Co₂₀Fe₆₀B₂₀ having a thickness generally rangingfrom, but not limited to, 0.8 nm-1.6 nm); a dielectric layer 316comprising a dielectric oxide such as MgO or Al₂O₃ having a thickness ofapproximately, but not limited to, 0.8-1.4 nm); a FM fixed layer 318preferably comprising a Cobalt-rich CoFeB material (e.g. Co₆₀Fe₂₀B₂₀having a thickness of approximately, but not limited to, 2.7 nm); ametal layer (e.g. Ru 320 having a thickness of approximately, but notlimited to, 0.85 nm) to provide antiferromagnetic inter-layer exchangecoupling; an exchange-biased layer 322 of Co_(m)Fe₃₀ (e.g., thickness ofapproximately, but not limited to, 2.3 nm), the magnetizationorientation of which is pinned by exchange bias using ananti-ferromagnetic layer 324, e.g. PtMn, IrMn, or a like material havinga thickness of approximately, but not limited to, 20 nm); and a topelectrode 326. By way of example and not limitation, the pillar of thedevice depicted is in the shape of a 170 nm×60 nm elliptical nanopillar.In this illustration, Ta layer 312 is used as a seed layer to helpinduce a larger magnitude of electron spin and/or enhance theelectric-field sensitivity of magnetic properties (such as anisotropy)in the FM free layer. It also acts as a sink of B atoms during annealingof the material stack after deposition, resulting in bettercrystallization of the FM free layer and thereby increasing the TMReffect. Of course any suitable materials can be used as a capping orseed layer 312; for example, materials based on Ruthenium, hafnium, andpalladium, may be used. More generally, any adjunct layers that can helpfacilitate the proper functioning of the MEJ can be implemented inaccordance with embodiments of the invention.

In numerous embodiments, the MEJ includes a semi-fixed layer which has amagnetic anisotropy that is altered by the application of a potentialdifference. In many instances the characteristic magnetic anisotropy ofthe semi-fixed layer is a function of the applied voltage. For examplein many cases, the direction of the orientation of the magneticanisotropy of the semi-fixed layer is oriented in the plane of the layerin the absence of a potential difference across the MEJ. However, when apotential difference is applied, the magnetic anisotropy is altered suchthat it includes a strengthened out-of-plane anisotropy. Moreover, theextent to which the magnetic anisotropy of the semi-fixed layer ismodified as a function of applied voltage can be made to be less thanthe extent to which the magnetic anisotropy of the FM free layer ismodified as a function of applied voltage. The incorporation of asemi-fixed layer can facilitate a more nuanced operation of the MEJ (tobe discussed below).

FIG. 4 illustrates an MEJ that includes a semi-fixed layer. Inparticular, the configuration of the MEJ 400 is similar to that depictedin FIG. 1, insofar as it includes an FM fixed layer 402 and an FM freelayer 404 separated by a dielectric layer 406. However, the MEJ 400further includes a second dielectric layer 408 adjoined to the FM freelayer 404 such that the FM free layer is adjoined to two dielectriclayers, 406 and 408 respectively, on opposing sides. Further, asemi-fixed layer 410 is adjoined to the dielectric layer. Typically, thedirection of magnetic polarization of the semi-fixed layer 414 isantiparallel with that of the FM fixed layer 412. As mentioned above,the direction of magnetic polarization of the semi-fixed layer can bemanipulated based on the application of a voltage. In the illustratedembodiment, it is depicted that the application of a potentialdifference adjusts the magnetic anisotropy of the semi-fixed layer suchthat the strength of the magnetic anisotropy along a directionorthogonal to the initial direction of magnetization polarization isdeveloped. Note that in the illustrated embodiment, the directions ofmagnetic polarizations are all depicted to be in-plane where there is nopotential difference. However, of course it should be understood thatthe direction of the magnetic polarization can be in any suitabledirection in accordance with embodiments of the invention. Moregenerally, although a particular configuration of an MEJ that includes asemi-fixed layer is depicted, it should of course be understood that asemi-fixed layer can be incorporated within an MEJ in any number ofconfigurations in accordance with embodiments of the invention. Forexample, FIG. 4B illustrates an MEJ that includes a semi-fixed layerthat is in a different configuration than that seen in 4A. Inparticular, the MEJ 450 is similar to that seen in FIG. 4A, except thatthe positioning of the semi-fixed layer 464 and the free layer 454 isinverted. In certain situations, such a configuration may be moredesirable.

Indeed, although several depictions of particular MEJs suitable forimplementation within a DIOMEJ cell have been described, it should ofcourse be understood that any of a variety of MEJ cells can beincorporated within a DIOMEJ cell in accordance with embodiments of theinvention. For example, any suitable MEJ disclosed in InternationalPatent Application Number PCT/US2012/038693, cited above, can beimplemented

The general operation of an MEJ is now discussed.

General Principles of MEJ Operation

In numerous embodiments, MEJs are utilized in DIOMEJ cells based upontheir operating principles. MEJs generally function to achieve twodistinct information states using the principles of magnetoresistance.As mentioned above, magnetoresistance principles regard how theresistance of a thin film structure that includes alternating layers offerromagnetic and non-magnetic layers depends upon whether theferromagnetic layers are in a parallel or antiparallel alignment. Thus,an MEJ can achieve a first information state where its FM layers havemagnetic polarizations that are parallel, and a second information statewhere its FM layers have magnetic polarizations that are antiparallel.MEJs further rely on the principles of voltage controlled magneticanisotropy (VCMA). Generally, VCMA principles regard how the applicationof a potential difference across a ferromagnetic material that isadjoined to a dielectric layer can impact the characteristics of itsmagnetic anisotropy. For example, it has been demonstrated that theinterface of oxides such as MgO with metallic ferromagnets such as Feand CoFeB can exhibit a large perpendicular magnetic anisotropy which isfurthermore sensitive to voltages applied across the dielectric layer,an effect that has been attributed to spin-dependent charge screeningand to the electric field induced modulation of the relative occupancyof atomic orbitals at the interface. In any case, based on theseprinciples, MEJs can achieve two distinct information states. Generally,MEJs can employ two mechanisms to do so: first, MEJs can be configuredsuch that the application of a potential difference across the MEJfunctions to reduce the coercivity of the FM free layer, such that itcan be subject to magnetization in a desired polar direction, i.e.either parallel with or antiparallel with the polarization direction ofthe fixed layer; second, MEJ operation can rely on precessionalswitching (or resonant switching), whereby by precisely subjecting theMEJ to voltage pulses of precise widths, the direction of magneticpolarization of the FM free layer can be made to switch.

In many embodiments, MEJ operation is based on reducing the coercivityof the FM free layer such that it can be magnetized in a desireddirection. With a reduced coercivity, the FM free layer can bemagnetized in any suitable way in accordance with embodiments of theinvention. For instance, the magnetization can result from an externallyapplied magnetic field, the magnetization field resulting from the FMfixed layer, the application of a spin-transfer torque (STT) current,the magnetization field resulting from a FM semi-fixed layer, anycombination of these mechanisms, or any suitable method of magnetizingthe FM free layer with a reduced coercivity.

By way of example and not limitation, examples of suitable ranges forthe externally applied magnetic field are in the range of 0 to 100 Oe,with preferred embodiments working without an externally applied field.The magnitude of the electric field applied across the device to reduceits coercivity or bring about resonant switching can be approximately inthe range of 0.1-2.0 V/nm, with lower electric fields required formaterials combinations that exhibit a larger VCMA effect. The magnitudeof the STT current used to assist the switching may be in the range ofapproximately 0.1-1.0 MNcm².

FIG. 5A depicts how the application of a potential difference can reducethe coercivity of the free layer such that it can be magnetized by anexternally applied magnetic field H. In the illustrated embodiment, instep 1, the FM free layer and the FM fixed layer have a magneticpolarization that is substantially in plane; the FM free layer ismagnetized in a direction that is parallel with that of the FM fixedlayer. Further, in Step 1, the coercivity of the FM free layer is suchthat the FM free layer is not prone to having its direction of magneticpolarization reversed by the magnetic field H, which is in a directionantiparallel with the polarization direction of the FM fixed layer.However, a Voltage, V_(c) is then applied, which results in step 2,where the voltage V_(c) has modified the magnetic anisotropy of the freelayer such that the strength of the magnetic anisotropy along an easyaxis that is orthogonal to the initial easy axis is magnified.Correspondingly, the coercivity of the FM free layer is reduced suchthat it is subject to magnetization by an in-plane magnetic field H.Accordingly, when the potential difference V_(c) is removed, VCMAeffects are removed and the magnetic field H magnetizes the FM freelayer in a direction that is antiparallel with the polarization of theFM fixed layer. Hence, as the MEJ now includes an FM fixed layer and anFM free layer that have magnetic polarizations that are antiparallel, itreads out a second information state (resistance value) different fromthe first. Thus, it can be seen that by controlling the potentialdifference and the direction of an applied magnetization, an MEJ switchcan be achieved.

It should of course be understood that the direction of the FM fixedlayer's magnetic polarization need not be in-plane—it can be in anysuitable direction in accordance with embodiments of the invention. Forinstance, it can be substantially out of plane. Additionally, the FMfree layer can include magnetic anisotropies that are both in-plane andout-of-plane; indeed, in many instances, it has been observed that thecoercivity is most sensitive to the application of voltage when thein-plane anisotropy and out-of-plane anisotropy are of relativelysimilar strengths. FIG. 5B depicts a corresponding case relative to FIG.5A when the FM fixed and FM free layers have directions of magneticpolarization that are perpendicular to the layers of the MEJ(out-of-plane). It is of course important, that an FM, magneticallyanisotropic, free layer be able to adopt a magnetic polarizationdirection that is either substantially parallel with an FM fixed layer,or substantially antiparallel with an FM fixed layer. In other words,when unburdened by a potential difference, the easy axis of the FM freelayer should be aligned with the direction of magnetic polarization,such that the FM free layer can adopt a direction of polarization thatis either parallel with or antiparallel with the direction of the FMfixed layer's polarization, to the extent that a distinct measurabledifference in the resistance of the MEJ that results from the principlesof magnetoresistance as between the two states (i.e. parallel alignmentvs. antiparallel alignment) can be measured, such that two distinctinformation states can be defined.

Note of course that the application of an externally applied magneticfield is not the only way for the MEJ to take advantage of reducedcoercivity upon application of a potential difference. For example, inmany embodiments, the magnetic polarization of the FM fixed layer isused to magnetize the free layer when it has a reduced coercivity.Moreover, in a number of embodiments, an MEJ is configured to receive aspin-transfer torque (STT) current when application of a voltage causesa reduction in the coercivity of the FM free layer. Generally, STTcurrent is a spin-polarized current that can be used to magnetize amagnetizable layer. Accordingly, the STT current can then magnetize theFM free layer, where the direction of the spin determines the directionof magnetization. This configuration is advantageous over conventionalSTT-RAM configurations since the reduced coercivity of the FM free layerreduces the amount of current required to magnetize the FM free layer,thereby making the device more energy efficient.

Additionally, in many embodiments, the MEJ cell further takes advantageof thermally assisted switching (TAS) principles. Generally, inaccordance with TAS principles, heating up the MEJ during a writingprocess reduces the magnetic field required to induce switching. Thus,for instance, where STT is employed, even less current may be requiredto magnetize a free layer, particularly where VCMA principles have beenutilized to reduce its coercivity.

Moreover, the switching of MEJs to achieve two information states canalso be achieved using voltage pulses. In particular, if voltage pulsesare imposed on the MEJ for a time period that is one-half of theprecession of the magnetization of the free layer, then themagnetization may invert its polarity. Using this technique, ultrafastswitching times, e.g. below 1 ns, can be realized; moreover, usingvoltage pulses as opposed to a current, makes this technique moreenergetically efficient as compared to the precessional switchinginduced by STT currents, as is often used in STT-RAM. However, thistechnique is subject to the application of a precise pulse that is halfthe length of the precessional period of the magnetization layer. Forinstance, it has been observed that pulse durations in the range of 0.05to 3 nanoseconds can reverse the magnetic polarization. Additionally,the voltage pulse must be of suitable amplitude to cause the desiredeffect, e.g. reverse the direction of magnetic polarization.

With these principles in mind, the unipolar operation of MEJs is nowdiscussed.

Unipolar MEJ Operation

In many embodiments, the MEJ is configured so that a voltage of a singlepolarity can allow the MEJ to adopt either of the two specifiedinformation states. This can be achieved using any of a number ofconfigurations. For example, an STT current can be used in conjunctionwith an applied magnetic field to allow an MEJ to switch informationstates using voltages of a single polarity. For example, in someembodiments, the MEJ is subject to a biasing magnetic field. When theMEJ is subject to a potential difference, V₁, the coercivity of the FMfree layer is reduced, such that the biasing magnetic field canmagnetize the FM free layer in the biased direction. This aspect of theoperation is similar to that depicted in FIGS. 5A and 5B. However, whenthe MEJ is further subject to an increased voltage V₂ (i.e., increasedin magnitude), the coercivity is further reduced; at this greaterpotential difference, current-induced effects become important. Inparticular, the greater voltage induces a current that can be made to bean STT current and thereby magnetize the free layer. The direction ofpolarization that the STT current induces can be antiparallel with thatof the magnetic field. Thus, at a voltage V₂, the induced STT currentplays a more prominent role in magnetizing the FM free layer and doesso. Ultimately, at a voltage V₁, the biasing field determines themagnetization of the free layer; whereas, at a voltage V₂, the STTcurrent determines the magnetization of the free layer. Accordingly,each of two distinct information states can be established usingvoltages of a single polarity.

In many embodiments, an external biasing magnetic field is not reliedupon in this configuration; instead the magnetic field imposed by the FMfixed layer is used to establish direction of polarization at the lowervoltage V₁.

In some embodiments, MEJs further include a semi-fixed layer that has amagnetic anisotropy that can be altered by the application of apotential difference, e.g., as depicted in FIG. 4. Thus, in manyembodiments, when a relatively low voltage V₁, is applied to the MEJthat includes a semi-fixed layer, the magnitude of the voltage is notsufficiently strong to noticeably impact the magnetic anisotropy of thesemi-fixed layer. Accordingly, the semi-fixed layer still impacts theoverall magnetic field. However, when a relatively greater voltage V₂that exceeds the threshold voltage required to noticeably alter themagnetic anisotropy of the semi-fixed layer is applied to the MEJ, theeffect of the magnetic polarization of the semi-fixed layer on theoverall magnetic field is mitigated or reduced. Accordingly, thisphenomenon can be exploited to achieve an MEJ that can achieve twoinformation states using voltages of a single polarity. For example, inmany embodiments, where a relatively lower voltage, V₁, is applied, thecoercivity of the free layer is reduced making it susceptible tomagnetization, and the semi-fixed layer plays a dominant role inmagnetizing the free layer. However, when a relatively higher voltage,V₂, is applied, the magnetizing ability of the semi-fixed layer ismitigated, and the fixed layer plays a dominant role in establishing thepolarization of the free layer. Of course, as before, an externallyapplied biasing magnetic field can be implemented consistent with theseprinciples.

Although several examples are provided for achieving unipolar operationof an MEJ, any number of configurations can be implemented for unipolaroperation of an MEJ, in accordance with embodiments of the invention.For example, in a number of embodiments, a voltage of a single polarityis used to reduce the coercivity of the FM free layer, and either of twodifferent magnetic fields, oriented either parallel with or antiparallel with, a respective FM fixed layer are used to write to the MEJand define information states. Thus, the above-described examples forrealizing unipolar operation are meant to be illustrative and notcomprehensive.

Importantly, where voltages of a single polarity are used to write tothe MEJ, then a diode may be used as an access device and coupled to theMEJ to form a DIOMEJ cell. The arrangement of a DIOMEJ cell is nowdiscussed below.

Diomei Cell

In many embodiments, a DIOMEJ cell is realized by coupling a diode to aMEJ cell and using the diode as an access device. In particular, whereasconventionally MEJ cells utilize transistors as access devices, using adiode can confer many advantages. In particular, diodes can be made toconform to a smaller form factor. Accordingly, a DIOMEJ cell can be madeto be more densely packed as compared to a MEJ—transistor configuration.Moreover, the implementation of a diode can reduce the occurrences ofsneak currents. Diodes can be coupled to an MEJ in any suitable fashion,and any suitable diode may be used. For example, FIG. 6 depicts a DIOMEJthat incorporates a Schottky diode. In particular, in the illustratedembodiment, a DIOMEJ cell, 600, includes a Schottky diode, 602, that iscoupled to the FM fixed layer of an MEJ 604. The metal-semiconductorinterface junction region creates a Schottky diode barrier that hasproperties of a diode, but the switching speed is generally fast, suchas in the picoseconds range, and the forward voltage drop is relativelysmall, such as from 0.1 to 0.4 Volts. Of course, although a DIOMEJ cellincorporating a Schottky diode is illustrated, any suitable diode can beimplemented in a DIOMEJ cell in accordance with embodiments of theinvention. For example, a zener diode, an avalanche diode, and a tunneldiode, can be incorporated in a DIOMEJ cell in accordance withembodiments of the invention.

Moreover, a ‘diode-type’ device may be coupled to an MEJ to form aDIOMEJ cell. For example, FIG. 7 depicts a DIOMEJ cell that includes a‘diode-type’ device. In particular, the DIOMEJ 700 includes adiode-connected transistor 702 that is coupled to the FM fixed layer ofan MEJ 704. The utilization of a diode-connected transistor can offermore flexibility. For example, when transistor 702 is a NMOS, its widthand length sizes are often adjustable to yield a particular resistancevalue when it is in the OFF state and a lower value when it is in the ONstate. As a diode-connected NMOS 702, the gate and drain of NMOS 702 areelectrically connected together, and the source node of NMOS 702 iselectrically connected in series to the MEJ. Alternatively, transistor702 is a diode-connected PMOS transistor, a bipolar or a JFET device,and so on. As a diode-connected PMOS, the drain node of transistor 702is electrically connected in series to the MEJ.

Of course, it should be understood that it is not requisite that a diodeor diode-type device be connected to the FM fixed layer of an MEJ;indeed, in many embodiments, a DIOMEJ cell includes a diode electricallycoupled to a FM free layer of an MEJ. FIG. 8 illustrates a DIOMEJ cell,800, where a diode 802 is electrically coupled to the free layer of anMEJ 804. Any suitable way of electrically coupling a diode to an MEJsuch that it can act as an access device may be implemented inaccordance with embodiments of the invention.

Although not drawn to scale, FIG. 9 provides a 3D schematic of a DIOMEJcell. In particular, the DIOMEJ cell 900 includes a diode 902 inelectrical communication with an MEJ 904. Interconnect contacts betweendiode 902 and MEJ 904 are not shown and in some embodiments there is nointerconnect, stub, via, contact, wire or electrode, altogether, if alayer of the MEJ 904 serves dual purposes, for both electricalinterconnection and for performing its expected magnetizing abilities.In the illustrated embodiment, diode 902 and MEJ 904 are both generallycylindrical and may share the same central axis 906. The dimensions ofthe configuration can vary. For example, in many embodiments, theoverall vertical dimension is in the range of 20 nm-100 nm for the diode902, and 20 nm-70 nm for MEJ 904. In a number of embodiments, thediameter for diode 902 is typically in the range of 10 nm-150 nm and forMEJ 904 is 10 nm-180 nm. In an embodiment where it is desirable toincrease the OFF state resistance of the diode, its diameter should bereduced and/or its vertical length should be increased. Also, when aDIOMEJ cell 900 is part of an array of DIOMEJ cells, thecenter-to-center spacing between adjacent DIOMEJ cells 900 are typicallyon the order of 20 nm to 2000 nm, and are determined by the desiredarray density and layout rules for the given manufacturing technology.Moreover, in many embodiments, there is insulating material such asoxide or nitride between adjacent DIOMEJ cells 900. Although specificdimensions have been recited, it should of course be realized thatDIOMEJ cells can be implemented in a variety of dimensions in accordancewith embodiments of the invention, and the dimensions are not limited tothe recitations.

Although FIG. 9 depicts a generally cylindrical or pillar-shaped (ornanopillars) DIOMEJ cell 900, other geometries are possible, dependingon the application, on the packing density of the DIOMEJ cell, or on themanufacturing tool capability available. For example, FIG. 10 depicts acircular top view of the DIOMEJ cell 1000, or a bottom view if the diode1002 were fabricated before the MEJ 1004. In FIG. 10, the diameter ofdiode 1002 is smaller than that of MEJ 1004, but alternatively to avoidetch effects, in other embodiments the diameter of the diode 1002 ismade comparable to that of MEJ 1004 so that the two cross sectionalareas may coincide, or the diode 1002 may be larger.

Moreover, although the cross sectional area of either the diode 1002 orMEJ 1004 is depicted as being substantially circular, other shapes arepossible or even desirable based on the circumstances. For example, forin-plane oriented spin operation of an MEJ, an oval or elliptical crosssectional area as shown in FIG. 11A is often more optimal for the MEJ1104. The ratio of the long diameter to the short diameter is typicallyin the range of 3.5 to 1.5; but of course, it can be in any suitablerange.

In another embodiment, the cross-sectional areas of both devices arerectangular or substantially square, or with the corners beveled orrounded so that the cross sectional area is hexagonal or octagonal asshown in FIG. 11B. The illustrated embodiment depicts a DIOMEJ cell 1130including an MEJ 1134 and a diode 1132 that are both hexagonal in shape.Note that beveled or rounded corners may reduce charge congregation,micromagnetic texture, and large local electromagnetic fields.Cross-sectional geometries need not coincide. For example, FIG. 11Cdepicts a DIOMEJ cell that implements two different cross sections. Inparticular, the DIOMEJ cell 1160 includes an MEJ 1164 and a diode 1162that have differing cross sections; specifically, the MEJ 1164 has ahexagonal cross section, while the diode 1162 has a circularcross-section. Note that the geometries can be optimized inconsideration of the crystalline structure of the constituent materials.Moreover, the sizes and geometries of MEJ and diode are also furthertunable to provide a desired performance property such as resistivity.

Diomej cell applications are discussed below.

Diomei Cell Applications

Because of their form-factor and their energy efficiency, DIOMEJ cellsare particularly versatile and can be implemented in a host ofapplications. For example, DIOMEJ cells can be used as simple switches,incorporated in logic circuits, and used as a fundamental element in aMeRAM configurations.

FIG. 12 illustrates how DIOMEJ cells can be incorporated in a MeRAMconfiguration that employs a cross-bar architecture. In particular, theMeRAM crossbar array 1200 includes DIOMEJ cells 1201 that each have anMEJ 1204 and a diode 1202 in electrical communication. The illustrationshows a three by three array of DIOMEJ cells 1201, but of course itshould be understood that an array of DIOMEJ cells of any size can beimplemented in accordance with embodiments of the invention. In theillustrated embodiment, the anode of the diode 1202 is electricallycoupled to one of an array of bit lines 1250, while the MEJ 1204 iselectrically coupled to one of an array of source lines 1275. Althoughit should be understood that the MEJ 1204 can be coupled to an array ofbit lines, while the anode of the diode can be coupled to one of anarray of source lines. Of course, it is understood that the MEJs 1204are configured for unipolar operation such that the DIOMEJ cells 1201can function. Accordingly, new bits of information are written to an MEJ1201 (i.e. information states are established in the MEJ), where apotential difference is established across a respective bit line in thearray of bit lines 1250 and a respective source line in the array ofsource lines 1275. In this configuration, the diode 1202 acts as anaccess device. The use of diodes as access devices is advantageousinsofar as they can prevent parasitic paths that may be present in aMeRAM configuration, and can relatedly improve reading of the DIOMEJcells since the on/off resistance ratio will be improved. Note also,that the incorporation of diodes, instead of transistors, can allowDIOMEJ cells to be more densely packed, and thereby result in MeRAM withgreater capacity.

FIG. 13 depicts a stacked MeRAM configuration. In particular, the MeRAMconfiguration 1300 includes a substrate 1302, a layer of circuitrydeposited on the substrate 1304, and layers of arrays of DIOMEJ cellsconfigured to operate as memory deposited thereon 1306. The substratelayer 1302 simply functions as a structural base layer. The circuitry1304 can be any circuitry suitable for implementation in a MeRAMconfiguration, and can include for instance: transistors, addresscircuits, decode circuits, read and write circuits, logic gates, andsense amplifier circuits to control and operate the cross-bar stackedmemory array 1300. The ability to layer arrays of DIOMEJ cells isgreatly facilitated by the DIOMEJ cells form-factor, and the layeringcan allow for a densely packed memory configuration.

Moreover, in many embodiments, MeRAM configurations exploit redundantmemory bits that are used to record parity and to enable missing data tobe reconstructed by an error-correcting code (ECC). Parity allows thedetection of single-bit errors. One common error-correcting code, aSECDED Hamming code, allows a single-bit error to be corrected and, inthe usual configuration, with an extra parity bit, double-bit errors tobe detected. As DIOMEJ cells can be arranged relatively densely in aMeRAM configuration, the desire to incorporate redundant bits is not asburdensome in these configurations.

Diomej cells can also be incorporated in field programmable gate arrays(FPGAs) in accordance with embodiments of the invention. For example,DIOMEJ cells can be implemented in a hybrid FPGA that includesconfigurable logic blocks and embedded memory, along with other possiblefunctions such as a DSP, floating point units, etc. FIG. 14 illustratesa hybrid FPGA that includes DIOMEJ cells that can be implemented inaccordance with embodiments of the invention. In particular, the FPGA1400 includes configurable logic blocks 1402, and DIOMEJ cells that areconfigured to act as memory 1404 (e.g. MeRAM configurations, asdescribed above). The logic blocks 1402 can include look up tables thatinclude memory made from the DIOMEJ cross-bar memory arrays or stackedDIOMEJ memory arrays 1404. The embedded memory 1404 is placed and routedtogether and formed in the center, with the configurable logic blocks1402 located on the periphery of the embedded memory, 1404. Thisconfiguration eases interconnection (electrical wiring or traces)routing between an embedded memory block 1404, and an associated logicblock, 1402. Alternatively, to make the capacitive and resistive load ofthe electric wiring/traces more uniform between each pair of embeddedmemory block 1404 and associated logic block 1402, the embedded memoryblocks 1404 and logic blocks 1402 are interleaved and placed in acheckerboard or island pattern.

Upon startup, the FPGA 1400 is programmed so that its logic blocks 1402have the needed logic gates to perform certain functions. To ensuresecurity, instead of programming an FPGA from an external source where abit stream might be monitored and captured by an enemy or competitor orthief, if the embedded memory 1404 is compact and dense enough, multiplebit stream instructions may be stored, one on each embedded memory unit.Then, upon startup it would only be necessary to transmit one code toselect an appropriate algorithm that is stored in a particular embeddedmemory unit to program the FPGA 1400 to perform a particular function.Moreover, the FPGA configuration (i.e. the configuration in the logicblocks 1402) can be readily changed on the fly, while a mission is intransit or in operation, if all the possible algorithms of programmingthe FPGA 1400 are stored in the embedded memory 1404. This type ofapplication benefits from the non-volatile nature of the DIOMEJcross-bar memory arrays or stacked arrays. Due to its highly flexibleproperties, the hybrid FPGA can be used in products for security,communications, data processing, industrial plants and manufacturing,military and aerospace, consumer electronics and the entertainmentindustry, and automotive. Specific products include mobile phones,tablets, computers, digital cameras, digital audio players,synthesizers, video games, scientific instrumentation, industrialrobotics, medical electronics, smart weapons, laser-radars, un-mannedair vehicles and so on.

Fabrication

The foregoing discussion highlighted the power efficiency, speed, andnon-volatility of DIOMEJ cells and MEJs. These components are furtheradvantageous insofar as they possess favorable manufacturingcharacteristics. For instance their manufacture can result in a highyield, their manufacture is scalable, and the components are relativelydurable (e.g., they don't have intricate geometries) and can be subjectto more rigorous manufacturing processes. Their manufacture can be madeto be versatile as well. For instance, the manufacture of DIOMEJ cellsand MEJs can be adapted for the front end or the back end of thesemiconductor line process. For example, a Schottky diode can be formedin the front end of the semiconductor process line's doped semiconductorand metal layers. Fabricating a diode at the front end entails dopingthe semiconductor crystal by ion implantation, diffusion of dopants, orby epitaxy growth so that there are p-doped and n-doped semiconductorsto form a p-n junction. This is especially the case in bipolar, BCD,Bi-CMOS, or Bi-Com (complementary Bi-CMOS) technology. Subsequently, anMEJ can be formed on the back end of the line process. Of course metaland via layers can interconnect the components in a suitable fashion,and the development of these interconnects can occur at any suitablepoint during the fabrication process.

In many embodiments, entire DIOMEJ cells, and even arrays of DIOMEJcells, are formed in the back end of a semiconductor process line; thus,the diode is also fabricated in the back end.

An exemplary backend method for forming the integrated DIOMEJ cells isillustrated in FIGS. 15-21. FIG. 15 depicts a DIOMEJ cell 1500 on asilicon or silicon dioxide substrate 1508 or other wafer material.Although layer 1508 is labeled “substrate,” unless the DIOMEJ cells 1500are made as standalone objects on bare wafer, there can be transistors,devices, and circuits at that layer 1508, although they are notillustrated. The memory MEJ 1500 includes a fixed ferromagnetic, freeferromagnetic, dielectric, and can include other layers. For example,the layers of the MEJ, 1500, can include Co, Fe, CoFeB, and MgO. In oneexample, the free layer comprises Co₂₀Fe₆₀B₂₀; the dielectric barrierlayer comprises MgO; and the fixed layer comprises several layersCo₄₀Fe₄₀B₂₀, Ru, Co_(m)Fe₃₀, and PtMn. The diode 1502 includes twosemiconductor layers, or semiconductor and a metal layer depending onwhether it is a P-N junction or a Schottky diode, respectively. Forexample, the diode 1502 materials include doped silicon, polysilicon, oramorphous silicon. There is an optional metal layer 1506 in between thediode 1502 and MEJ 1504, which together form a DIOMEJ cell 1500. Bottomelectrode 1510 and top electrode 1512 allow electrical connection to theDIOMEJ cell 1500. For example the electrodes 1508 and 1512 connect tothe rest of the crossbar metallization. The electrodes 1510 and 1512 arelikely to be upper level metals because this is a back end of theprocess example of the method of forming the DIOMEJ cells 1500. Themetals include aluminum, copper, tungsten, hafnium, tantalum, thallium,ruthenium, or alloys of these metals, or other materials.

In many embodiments, an MEJ is prepared by depositing continuousmultiple layers of films of different material (e.g. CoFeB, MgO, PtMn,IrMn, synthetic anti-ferromagnetic material). For example, the films forthe fixed ferromagnetic layers and free ferromagnetic layers aredeposited by a physical vapor deposition (PVD) system and subsequentlyannealed in an in-plane or out-of-plane magnetic field, or without amagnetic field, above 200° C. Annealing may take place under vacuumconditions to avoid oxidation of the material stack. As further example,metallic films are deposited by DC frequency sputtering while thedielectric tunnel layer is deposited by radio-frequency sputtering froma ceramic MgO target, or by dc sputtering of Mg and subsequentoxidation, or by a combination of both. The sputtering is performed bymagnetron sputter deposition uniformly on a surface that is held atapproximately ambient temperatures. The surfaces of these various layersmay be planarized after each layer is formed, and the planarizationtechniques include chemical-mechanical polishing. The thickness of eachlayer is in the range of 0.1 to 10 nm, and is designed to achievecertain concentrations of spins or magnetization, resistivity, voltageranges to flip the spin, and various other electrical performanceparameters. For example, the dielectric tunnel layer is designed to bethick enough to make the current-induced spin-transfer torque small. Theswitching speeds in MEJs are adjusted based on their design andcomposition. As to the shape of the MEJ devices, depending on thematerial, the in-plane configuration tends to perform better if the flatend surface were elliptical, oblong, rectangular, etc., so that thegeometry is elongated in one direction (length is greater than thewidth). Otherwise, most of the MEJ devices have a more circular geometryon their ends, forming a nanopillar or column for the overall device.Typical lateral dimensions are smaller than 150 nm on each side, and maybe scaled down to as small as approximately 3 nm by engineering thematerial structures to allow for stable memory operation at thosedimensions.

In many embodiments, the diode is deposited after the MEJ material stackdeposition, using a chemical vapor deposition (CVD), physical vapordeposition (PVD), or any combination of these techniques, followed byetching into the DIOMEJ cell. Alternatively, the diode is depositedbefore the MEJ material stack deposition. These embodiments aredifferent from when the diode is fabricated on the front end and is thenincorporated as a separate device at a separate processing or depositionstage. Furthermore, in several embodiments, the MEJ in combination withdiodes are also possible if unipolar write voltages are used forflipping the spin in either direction, regardless of whether it has anout-of-plane or in-plane spin orientation.

In many embodiments, a manufacturing process flow for fabricating anentire array of DIOMEJ cells, for example for use in an MRAMconfiguration, is implemented. FIG. 16 illustrates an exemplary stage inthe manufacturing process flow for an entire array of DIOMEJ cells.After the appropriate materials for the layers of the DIOMEJ cells 1600and the electrodes 1610 and 1612 are formed, a hard mask layer 1614 isdeposited above the top electrode layer 1612. FIG. 17 depicts theresults after the individual DIOMEJ cells 1600 are defined byphotolithography or other patterning techniques, and then etched out(e.g. reactive ion etch “RIE”) and cleaned. FIG. 18 illustrates thedeposition of a spacer layer 1616 made of an insulator or dielectricmaterial. Photoresist 1618 or other light-sensitive material is applied.Then photolithography or other patterning defines the bottom endelectrode 1610. Etching techniques, such as RIE, expose the bottom endelectrodes 1610, as illustrated in FIG. 19. FIG. 19 also depicts thedeposition of oxide or nitride 1620 that may later be polished. FIG. 20illustrates the opening of the oxide or nitride 1620 to form a topelectrode 1622 (shown in FIG. 21) and possibly for bottom electrodes),followed by photolithography and via etch. In FIG. 21, the via openingis filled so that there is top electrode 1622 deposition, followed byphotolithography and etching. These top electrodes 1622 allowconnections between DIOMEJ cells 1600 to form the entire memory array orto connect to other circuits.

Thus, the crossbar array may be realized as a back end process (i.e.within the metallization layers) of a standard or custom CMOS (or othertransistor or semiconductor) process, and can thus be placed on top ofthe memory's peripheral and read/write circuitry. Multiple crossbararrays can be stacked in the back end processing, to achieve a largernumber of bits per area. Alternatively, upper levels of the stackedcrossbar arrays comprise memory arrays fabricated on bare wafers toproduce dies. Then the dice are stacked on top of one another andinterconnected to the rest of the circuits.

Generally, in many embodiments of the invention, DIOMEJ cells arefabricated by sequentially depositing their constituent layers onto asubstrate. Thus, for instance in some embodiments, a DIOMEJ cell isfabricated by first depositing layers of an MEJ on a substrate, andsubsequently depositing layers that constitute the diode. FIG. 22illustrates a process for fabricating a DIOMEJ cell that includes an MEJthat has a free layer, a fixed layer, and a dielectric layer by firstdepositing the layers of MEJ and then depositing the layers thatconstitute the diode. Generally, an electrode is deposited 2202 on asubstrate, and the electrode is then developed so that it possesses thedesired properties. Any suitable methods of treatment can be used todevelop the electrode, e.g. annealing, polishing, or any of the abovementioned treatments. A free layer of an MEJ is deposited 2204 on to thedeveloped electrode; it may also be developed so that it has the desiredproperties using any suitable technique. A dielectric layer is deposited2206 onto the developed free layer; it may also be developed so that ithas the desired properties using any suitable technique. A fixed layeris deposited 2208 onto the developed dielectric layer; it may also bedeveloped so that it has the desired properties using any suitabletechnique. A first diode layer is deposited 2210 onto the developedfixed layer; it may also be developed so that it has the desiredproperties using any suitable technique. A second diode layer isdeposited 2212 onto the developed first diode layer; it may also bedeveloped so that it has the desired properties using any suitabletechnique. An electrode layer is deposited 2214 onto the developed firstdiode layer; it may also be developed so that it has the desiredproperties using any suitable technique. Alternatively, some of thedevelopment steps may be combined or skipped as needed during thefabrication process. For example, if development of the layers involveshigh-temperature annealing, this may be done all at once afterdeposition of the entire stack, so that it affects the properties of alllayers, or alternatively, after deposition of the MEJ free, fixed anddielectric layers, but before deposition of the diode layers, such as toonly affect the properties of the MEJ layers. Of course, the illustratedprocess for manufacturing a DIOMEJ cell is meant to be illustrative. Itis of course to be understood that many variations of this process canbe implemented in accordance with embodiments of the invention. Forinstance, in many embodiments, the diode is electrically coupled to thefree layer as opposed to the fixed layer. In some embodiments, a layerof metal is installed in between the MEJ and the diode. In a number ofembodiments, the fixed and free layers include capping or seedmaterials. Thus many variations of the process of manufacturing DIOMEJcells can be implemented in accordance with embodiments of theinvention.

Finally, the orientation and directions stated and illustrated in thisapplication should not be taken as limiting. For example, thedirections, e.g. “top,” are merely illustrative and do not orient theembodiments absolutely. That is, a structure formed on its “side” or“bottom” is merely an arbitrary orientation in space that has noabsolute direction. Also, in actual usage, for example, a circuit maywell be on its “side” because circuit boards may be oriented in anydirection; and then, “top” is pointing to the “side.” Thus, the stateddirections in this application are arbitrary designations.

While certain features of the implementations have been illustrated anddescribed herein, modifications, substitutions, changes and equivalentswill occur to those skilled in the art. It is, therefore, to beunderstood that the claims are intended to cover all such modificationsand changes that fall within the scope of the embodiments. It should beunderstood that they have been presented by way of example only, notlimitation, and various changes in form and details may be made. Anyportion of the apparatus and/or methods described herein may be combinedin any combination, except mutually exclusive combinations. Theembodiments described herein can include various combinations and/orsub-combinations of the functions, components and/or features of thedifferent embodiments described. For example, other types ofvoltage-controlled magnetic junction devices may be substituted for thevoltage controlled MEJ's depicted and described herein. It is to beunderstood that the magnetoelectric elements and arrays can be utilizedin different embodiments and applications that may require tweaking tofit a particular situation and set of electronics. For instance,although the exemplary MEJs are described as having a fixed and freelayer, it is also possible to use a “three” layer embodiment, wherethere are fixed, free and semi-fixed layers. Such a different MEJ wouldalso be accompanied by a different range of voltages in order to performthe read and write operations. In addition, although the descriptionreferred most often to a MOS semiconductor process, other processes arepossible. For example, in the automotive industry, a unified bipolar,CMOS and high voltage DMOS/LDMOS process is common and can incorporatethe systems, devices and procedures described above. Bipolar, Bi-CMOS orBi-Com, BCD, MEMS, semi-Optical, RF, mixed-signal and other processesare all possible.

What is claimed is:
 1. A DIOMEJ cell comprising: a magnetoelectricjunction, that itself comprises: a ferromagnetic fixed layer; aferromagnetic, magnetically anisotropic, free layer; and a dielectriclayer interposed between said ferromagnetic fixed layer andferromagnetic, magnetically anisotropic, free layer; wherein theferromagnetic fixed layer is magnetically polarized in a firstdirection; wherein the ferromagnetic, magnetically anisotropic, freelayer has a first easy axis that is substantially aligned with the firstdirection, such that the ferromagnetic, magnetically anisotropic, freelayer can adopt a magnetic polarity that is either parallel with orantiparallel with the first direction; and wherein the magnetoelectricjunction is configured such that when a potential difference is appliedacross the magnetoelectric junction, the magnetic anisotropy of theferromagnetic, magnetically anisotropic, free layer is altered such thatthe relative strength of the magnetic anisotropy along a second easyaxis that is orthogonal to the first easy axis, or the easy plane wherethere is no easy axis that is orthogonal to the first easy axis, ascompared to the strength of the magnetic anisotropy along the first easyaxis, is magnified or reduced for the duration of the application of thepotential difference; and a diode; wherein the diode and themagnetoelectric junction are arranged in series.
 2. The DIOMEJ cell ofclaim 1, wherein the first direction coincides with an in-planedirection.
 3. The DIOMEJ cell of claim 1, wherein the first directioncoincides with an out-of-plane direction.
 4. The DIOMEJ cell of claim 1,wherein the coercivity of the ferromagnetic, magnetically anisotropic,free layer is reduced when a potential difference is applied across themagnetoelectric junction.
 5. The DIOMEJ cell of claim 4, wherein theapplication of a first threshold potential difference across theferromagnetic fixed layer and the ferromagnetic, magneticallyanisotropic, free layer reduces the coercivity of the ferromagnetic,magnetically anisotropic, free layer to an extent where the strength ofthe magnetic field imposed by the ferromagnetic fixed layer issufficient to magnetize the ferromagnetic, magnetically anisotropic,free layer.
 6. The DIOMEJ cell of claim 5, wherein the application of asecond threshold potential difference that is greater in magnitude thanthe first threshold potential difference causes a spin-transfer torquecurrent to flow through the magnetoelectric junction; wherein thespin-transfer torque current magnetizes the ferromagnetic, magneticallyanisotropic, free layer in a direction antiparallel with the firstdirection.
 7. The DIOMEJ cell of claim 1, wherein the ferromagneticfixed layer comprises one of: iron, nickel, manganese, cobalt, FeCoB,FeGaB, FePd, and FePt.
 8. The DIOMEJ cell of claim 1, wherein theferromagnetic, magnetically anisotropic, free layer comprises one of:iron, nickel, manganese, cobalt, FeCoB, FeGaB, FePd, and FePt.
 9. TheDIOMEJ cell of claim 1, wherein the dielectric layer comprises one of:MgO and Al₂O₃.
 10. The DIOMEJ cell of claim 1, further comprising anexternally applied magnetic field that is either parallel with orantiparallel with the magnetic polarization of the ferromagnetic fixedlayer, wherein the externally applied magnetic field has a strengthsufficient to magnetize the ferromagnetic, magnetically anisotropic,free layer when its coercivity is reduced with the application of apotential difference across the ferromagnetic fixed layer and theferromagnetic free layer.
 11. The DIOMEJ cell of claim 1, furthercomprising a seed layer.
 12. The DIOMEJ cell of claim 11, wherein theseed layer comprises Tantalum.
 13. The DIOMEJ cell of claim 1, furthercomprising: a second dielectric layer and a semi-fixed layer; whereinthe second dielectric layer is interposed between the ferromagnetic,magnetically anisotropic, free layer and the semi-fixed layer; whereinthe semi-fixed layer has a direction of magnetic polarization that isantiparallel with the direction of magnetic polarization of theferromagnetic fixed layer; and wherein, when a potential difference isapplied across the magnetoelectric junction, the magnetic anisotropy ofthe semi-fixed layer is altered such that the relative strength of themagnetic anisotropy along a third easy axis that is orthogonal to thefirst easy axis, or the easy plane where there is no easy axis that isorthogonal to the first easy axis, as compared to the strength of themagnetic anisotropy along the first easy axis, is magnified or reducedfor the duration of the application of a potential difference; whereinthe extent of this alteration is less than that of the ferromagnetic,magnetically anisotropic, free layer.
 14. The DIOMEJ cell of claim 1,wherein the application of a potential difference pulse that has aduration that coincides with half of the precessional period of theferromagnetic, magnetically anisotropic, free layer, or an odd multiplethereof, inverts the direction of magnetic polarization of themagnetoelectric junction.
 15. A magneto-electric random access memory,comprising: an array of DIOMEJ cells; wherein each DIOMEJ cellcomprises: a magnetoelectric junction, that itself comprises: aferromagnetic fixed layer; a ferromagnetic, magnetically anisotropic,free layer; and a dielectric layer interposed between said ferromagneticfixed layer and ferromagnetic, magnetically anisotropic, free layer;wherein the ferromagnetic fixed layer is magnetically polarized in afirst direction; wherein the ferromagnetic, magnetically anisotropic,free layer has a first easy axis that is substantially aligned with thefirst direction, such that the ferromagnetic, magnetically anisotropic,free layer can adopt a magnetic polarity that is either parallel with orantiparallel with the first direction; and wherein the magnetoelectricjunction is configured such that when a potential difference is appliedacross the magnetoelectric junction, the magnetic anisotropy of theferromagnetic, magnetically anisotropic, free layer is altered such thatthe relative strength of the magnetic anisotropy along a second easyaxis that is orthogonal to the first easy axis, or the easy plane wherethere is no easy axis that is orthogonal to the first easy axis, ascompared to the strength of the magnetic anisotropy along the first easyaxis, is magnified or reduced for the duration of the application of thepotential difference; and a diode; wherein the diode and themagnetoelectric junction are arranged in series; a plurality of sourcelines; and a plurality of bit lines; wherein each DIOMEJ cell iselectrically connected to a unique combination of a source line and abit line, such that no other DIOMEJ cell is connected to the same bitline and the same source line; and wherein a source line and a bit linecan be used to establish a potential difference across a particularDIOMEJ cell.
 16. The magneto-electric random access memory of claim 15,wherein for at least one DIOMEJ cell, the first direction coincides withan in-plane direction.
 17. The magneto-electric random access memory ofclaim 15, wherein for at least one DIOMEJ cell, the first directioncoincides with an out-of-plane direction.
 18. The magneto-electricrandom access memory of claim 15, wherein for at least one DIOMEJ cell,the application of a first threshold potential difference across theferromagnetic fixed layer and the ferromagnetic, magneticallyanisotropic, free layer reduces the coercivity of the ferromagnetic,magnetically anisotropic, free layer to an extent where the strength ofthe magnetic field imposed by the ferromagnetic fixed layer issufficient to magnetize the ferromagnetic, magnetically anisotropic,free layer.
 19. The magneto-electric random access memory of claim 18,wherein, for the at least one DIOMEJ cell, the application of a secondthreshold potential difference that is greater in magnitude than thefirst threshold potential difference causes a spin-transfer torquecurrent to flow through the magnetoelectric junction that magnetizes theferromagnetic, magnetically anisotropic, free layer in a directionantiparallel with the first direction.
 20. The magneto-electric randomaccess memory of claim 15, wherein for at least one DIOMEJ cell, thecoercivity of the ferromagnetic, magnetically anisotropic, free layer isreduced when a potential difference is applied across themagnetoelectric junction.
 21. The magneto-electric random access memoryof claim 15, wherein for at least one DIOMEJ cell, the application of apotential difference pulse that has a duration that coincides with halfof the precessional period of the ferromagnetic, magneticallyanisotropic, free layer, or an odd multiple thereof, inverts thedirection of magnetic polarization of the magnetoelectric junction.